Texas Instruments LM3448 -120VAC, 6W Isolated Flyback LED Driver Eval Board LM3448-120VFLBK/NOPB LM3448-120VFLBK/NOPB 데이터 시트
제품 코드
LM3448-120VFLBK/NOPB
in
OUT
P
P
! |
R
FLTR1
R
SNS
PWM
I-LIM
1.27V
ISNS
ASNS
GND
FLTR1
FLTR2
DIM
DIM DECODER
4.9V
Tri-State
50k
370k
C
FLTR2
C
FLTR1
1k
RAMP GEN.
5.9 kHz
3V
1V
1V
750 mV
125 ns
LEADING EDGE BLANKING
1V
Nearly a constant on-
time as the line varies
The PWM reference increases
as the line voltage increases.
As line voltage increases, the voltage across the
inductor increases, and the peak current increases.
LED Current
RAMP
Design Guide
By using the line voltage injection technique, the FLTR2 pin has the voltage wave shape shown in
on it with no TRIAC dimmer in-line. Peak voltage at the FLTR2 pin should be kept below 1.25V
otherwise current limit will be tripped. Capacitor C11 in conjunction with resistor R15 acts a filter for noise.
Using this technique a power factor greater than 0.95 can be achieved.
Using this technique a power factor greater than 0.95 can be achieved.
shows how a constant
on-time is maintained.
Figure 29. Typical Operation of FLTR2 Pin
Turns Ratio
The first step with an isolated design is to determine the transformer turns ratio. This can be an iterative
process that will depend on the specified operating conditions, maximum stresses allowed for the LM3448
SW FET and re-circulating diode as well as transformer core parameters. For many LM3448 flyback
designs, an integer turns ratio of 4 or 5 is a good starting point. The next step will be to verify that the
chosen turns ratio results in operating conditions that do not violate any other component ratings.
process that will depend on the specified operating conditions, maximum stresses allowed for the LM3448
SW FET and re-circulating diode as well as transformer core parameters. For many LM3448 flyback
designs, an integer turns ratio of 4 or 5 is a good starting point. The next step will be to verify that the
chosen turns ratio results in operating conditions that do not violate any other component ratings.
Duty Cycle Calculation
The AC mains voltage at the line frequency f
L
is assumed to be perfectly sinusoidal and the diode bridge
ideal. This yields a perfect rectified sinusoid at the input to the flyback. The peak nominal input voltage V
IN-
PK(NOM)
is defined in terms of the input voltage V
IN(NOM)
,
(3)
Duty cycle is calculated at the nominal peak input voltage V
IN-PK(NOM)
. Note that this is the duty cycle for
flyback operation at the boundary of continuous conduction mode (CCM) operation. In order to ensure that
the converter is operating in DCM, the primary inductance of the transformer will be adjusted lower (refer
to "Transformer" section).
the converter is operating in DCM, the primary inductance of the transformer will be adjusted lower (refer
to "Transformer" section).
(4)
Peak Input Current Calculation
Due to the direct line-injection, the flyback converter operates as a constant power converter. Therefore
average input power over one line cycle will approximately equal the output power,
average input power over one line cycle will approximately equal the output power,
(5)
16
AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver
SNOA554C – October 2011 – Revised May 2013
Copyright © 2011–2013, Texas Instruments Incorporated